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Message-ID: <718FE80422504B16B78D4E4488E2F062@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <245132.24289.qm@web28107.mail.ukl.yahoo.com>
In-Reply-To: <245132.24289.qm@web28107.mail.ukl.yahoo.com>
Date: Tue, 13 Oct 2009 23:10:53 +0100
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Subject: Re: LF: MOFMT ADSL noise
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Dear Pete, Paul-Henrik,

M0FMT wrote:
....> Well I can only conclude it is ADSL because it is just raising the 
noise floor it is very subtle not really identifiable like say TV LTB, it's 
a mush...>

I have problems with ADSL on 500kHz at my QTH - it sounds like "white noise" 
with no audible distinctive features I can detect. The noise can be about 
10 - 20dB over the band noise here over a fairly wide bandwidth in the MF 
range. It does not seem to be a problem at 136k, perhaps because this is in 
the guard band between upstream and downstream ADSL signals. It is easy to 
tell if you are experiencing noise from your own ADSL connection - 
disconnect the incoming phone line (probably just unplugging the modem from 
the phone socket will be enough) and there will be a reduction in noise 
level. Unfortunately, there also seems to be a significant level of similar 
noise here from other sources, probably my neighbors' ADSL via the overhead 
phone lines.

I have had some success with a noise-cancelling arrangement here, where the 
ADSL noise is sampled using a current transformer made by passing the 
incoming phone line through a ferrite core, and summed with the signal from 
loop antennas via a variable phase-shifting and gain-adjusting network, 
which is adjusted for a null in the noise. Also, moving the loop around the 
QTH shows some positions are better than others. Actually, life is a bit 
more complicated at this particular QTH, since there also seems to be 
wide-band noise originating as very low-level sidebands from the local 
broadcast stations. This requires a second noise-cancelling network to null 
noise from that direction. Also, it was necessary to insert a relay in 
series with the loading coil to disconnect the TX antenna on receive, 
otherwise this coupled more noise into the receiving antennas. But when it 
is all adjusted properly, the overall noise level can be reduced in 
favourable directions by about 20dB, which is worth doing.

Cheers, Jim Moritz
73 de M0BMU